Previous research has shown that the Tl-Ca-Ba-Cu-O quinary system contains a large number of superconducting oxides. By varying the starting cation compositions and the processing conditions, at least five phases with superconducting transition temperatures (T.sub.c) above liquid nitrogen temperature (77 K) were identified: Tl.sub.1 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.x (Tl-1122), Tl.sub.1 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x (Tl-1223), Tl.sub.2 Ba.sub.2 Cu.sub.1 O.sub.x (Tl-2021), Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.x (Tl-2122), and Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x (Tl-2223). Articles describing the discovery and identification of these phases include: Z. Z. Sheng et al., Nature, Vol. 332, p. 55, 1988; Z. Z. Sheng et al., Nature, Vol. 332, p. 138, 1988; R. M. Hazen et al., Phys. Rev. Lett., Vol. 60, p. 1657, 1988; S. S. P. Parkin et al., Phys. Rev. Lett., Vol. 60, p. 2539, 1988; S. S. P. Parkin et al., Phys. Rev. Lett., Vol. 61, p. 750, 1988; and R. Beyers et al., Appl. Phys. Lett., Vol. 53, p. 432, 1988.
The superconducting transition temperature (T.sub.c) of Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x (Tl-2223) remains the highest yet found, i.e., 125 K. Like all superconducting oxides, the superconducting properties observed in the thallium materials at low temperatures depend critically on how the materials are processed at high temperatures. Processing studies to date have found that the phases that are formed depend on the starting composition, the use of an open or closed reactor, the annealing treatment, and the Tl.sub.2 O pressure. Articles describing the preparation of Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x superconductors include the following: W. Y. Lee et al., Appl. Phys. Lett., Vol 53, p. 329, 1988; W. Y. Lee et al., Physica C, Vol. 160, p. 511, 1989; M. Hong et al., Thin Solid Films, Vol. 181, p. 173, 1989; M. Kikuchi et al., Jpn. J. Appl. Phys., Vol. 28, p. L-382, 1989; S. Narain et al., Supercond. Sci. Technol., Vol. 2, p. 236, 1989; N. L. Wu et al., Physica C, Vol. 161, p. 302, 1989; J. J. Ratto et al., Jpn. J. Appl. Phys., Vol. 29, p. 244, 1990; and T. L. Aselage et al., J. Am. Ceram. Soc., Vol. 73, p. 3345, 1990. Additionally, Engler et al., U.S. Pat. No. 4,870,052, issued Sep. 26, 1989, discloses a method for producing stable, bulk Tl-Ca-Ba-Cu-O superconductors. These studies indicate that relatively high temperatures are required to form the Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x superconductor, above approximately 860.degree. C. in open systems and above approximately 890.degree. C. in closed systems.
The inventors herein previously reported the preparation of Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x superconductor films with T.sub.c 's as high as 120 K in application Ser. No. 07/647,382, filed Jan. 29, 1991. Films were deposited onto MgO, SrTiO.sub.3, LaAlO.sub.3, and yttria-stabilized ZrO.sub.2 substrates at ambient temperature in a symmetrical RF diode sputtering system using a pair of identical sputtering targets. A schematic of the deposition system is shown in FIG. 1. Targets 10 and 12 were separated by approximately 25 mm and placed directly opposite one another. A substrate 14 was positioned parallel to the center line of targets 10 and 12, but outside the discharge, to minimize resputtering of the film. The as-deposited films were not superconducting and required an ex-situ anneal at 890.degree.-900.degree. C. to make them superconductors. For the ex-situ anneal, the as-deposited films were sandwiched between fresh pellets of bulk Tl-2223, wrapped in gold foil, and sealed in a quartz tube initially containing 1 atmosphere oxygen. The Tl-2223 pellets were used to establish a thallium activity in the film that was within the Tl-2223 phase stability range, while the gold foil and sealed quartz tube were used to minimize thallium loss (as Tl.sub.2 O vapor) during the anneal. Tl-2223 films deposited onto MgO, SrTiO.sub.3, and yttria-stabilized ZrO.sub.2 substrates exhibited T.sub.c 's as high as 120 K, but their critical current densities were rather low (10.sup.5 A/cm.sup.2 at 6.5 K, 10.sup.4 A/cm.sup.2 at 77 K) and decreased rapidly in an applied magnetic field. This was due in part to the high processing temperature (890.degree.-900.degree. C.) needed to form the Tl-2223 phase, which produced interdiffusion and reaction with the substrates, decreased the intergranular coupling between Tl-2223 grains, and yielded rough films.
A key processing variable that is commonly ignored in the prior art is control of the oxygen partial pressure during synthesis. It will become clear from the discussion in the Detailed Description section provided below that there are important benefits to forming thallium superconductors in low oxygen pressures. In particular, low oxygen pressures enable the synthesis of Tl-2223 at a reduced processing temperature which, in turn, improves the critical current density of these thallium superconductors. The synthesis of the Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x superconductor at a reduced oxygen pressure is avoided in the prior art because to those of ordinary skill in the art, previous studies have shown that thallium loss increases in low oxygen pressures. Specifically, D. E. Morris et al., Physica C, Vol. 175, p. 156, 1991, disclose the use of high oxygen pressures to suppress thallium loss during Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.x (Tl-2122) synthesis.
One exception to this general practice in the prior art is found in Europe patent application, 0 303,249 A2. In that application, Uno et al. describe a two-step heat treatment for preparing bulk superconductors or superconducting powders that have carbon contents less than 0.1 weight %. The first step is to calcine starting compounds of the constituent cations in hydrogen, carbon monoxide, or an oxygen pressure less than 50 torr (6.6.times. 10.sup.-2 atmosphere), with less than 5 torr (6.6.times.10.sup.-3 atmosphere) oxygen pressure being preferred. The second step is to sinter the material at a higher temperature in a moderate oxygen pressure, typically one atmosphere. Uno et al. developed this two-step heat treatment in order to reduce the residual carbon content in YBa.sub.2 Cu.sub.3 O.sub.6+x to less than 0.1 weight %, because carbon contamination is believed to lower the critical current density of YBa.sub.2 Cu.sub.3 O.sub.6+x. However, Uno et al. also report several examples where the two-step heat treatment is used to prepare thallium or bismuth superconductors in bulk or powder form. For the thallium examples, there is no recognition of the need to prevent thallium loss during the heating steps. Indeed, it is not clear if the experiments were carried out in a closed or open system. The temperature-oxygen pressure combinations Uno et al. used in the first heating step will produce substantial thallium losses and are far below what has been found to be necessary for making high-quality Tl-2223. Any potential benefits from the reduced temperatures used in the first heating step are negated by the second heating step at high temperature and moderate oxygen pressure. Moreover, the temperature-oxygen pressure combinations used in the second heating step produce poor quality thallium superconductors. In some cases, they are above the Tl-2223 stability limit. Due to the use of a process with a two-step heat treatment, with the second heat treatment being conducted at high temperature, the benefit of applying controlled low oxygen pressure to reduce the maximum processing temperature is ignored and therefore not disclosed. This again leads to poor superconducting performance, i.e. a low critical current density of approximately 10.sup.3 -10.sup.4 A/cm.sup.2 in a bulk material produced by this method.
Furthermore, to prevent thallium loss, a closed reactor such as that shown in FIG. 1 is often used in the prior art to form the Tl-2223 superconductor. In a closed reactor, like the quartz ampoules used for thin film synthesis, the oxygen pressure increases as the samples are brought up to the annealing temperature. It will become clear from the discussion provided in the Detailed Description section below that as a result of the pressure increase, the temperature required to form Tl-2223 in a closed reactor is higher than that required in an open system if the closed reactor is initially filled with air or 1 atmosphere oxygen. The higher processing temperature in a closed reactor adversely affects the critical current density of the Tl-2223 superconductor.
The prior art has not recognized that lower oxygen pressures can be used to form thallium superconductors and therefore has not been able to lower the processing temperatures. In turn, the thallium superconductors of the prior art have not exhibited improved properties such a high critical current densities and low surface impedance.
It is therefore an object of the present invention to teach a method of forming Tl-2223 superconductors with a controlled oxygen partial pressure while maintaining minimum thallium loss, thus enabling those skilled in the art to overcome the prior art limitations discussed above.
Furthermore, it is an object of the present invention to provide a method to improve the synthesis process for producing Tl-2223 superconductors with higher T.sub.c, higher critical current density, and lower surface impedance.
It is another object of this invention to teach an improved method for producing bulk Tl-2223 superconductors from stoichiometric starting material, i.e., a Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 cation composition.
It is another object of this invention to teach a method for decreasing the processing temperature required to form single phase Tl-2223 in thin film or bulk form by controlling the oxygen partial pressure to yield a superconducting composition having a minimum of Tl-2122 impurity.
It is another object of this invention to teach a method of forming essentially single-phase Tl-2223 in thin film or bulk form by controlling both the process temperature and the oxygen pressure.
It is another object of this invention to provide Tl-2223 superconductors which have purities in excess of about 80-90%, with T.sub.c in excess of 120 K and high critical current densities.
It is yet another object of this invention to provide electrical devices using Tl-2223 superconductors having enhanced purity and enhanced superconducting properties.
It is another object of this invention to provide a lower temperature process for producing Tl-2223 superconductors having T.sub.c .gtoreq.120 K and purity at least about 80%.
It is another object of this invention to provide thallium superconductor films which have critical current densities that are less strongly affected by external magnetic fields than prior art thallium superconductor films.
It is another object of this invention to provide thallium superconductors having higher critical densities and lower surface impedance than prior art thallium superconductors.